1. Field of the Invention
The present invention relates to a mobile communication network having mobile communication terminal units and base stations that correspond to a plurality of mobile communication systems including a mobile satellite communication system.
2. Description of the Related Art
In a conventional mobile communication network that integrates a plurality of mobile communication systems including a mobile satellite communication system or the like, each terminal unit has hardware portions corresponding to the individual communication systems. Each mobile communication terminal unit monitors signal levels of signals received from base stations corresponding to individual communication systems, determines whether or not the terminal unit is in a service area of a higher priority system, and switches the current system to the higher priority system. When the mobile communication terminal unit monitors signal levels, it should use different hardware portions corresponding to the individual communication systems.
Referring to FIG. 6, a mobile communication terminal unit 1' operates corresponding to two mobile communication systems that are a cellular system and a mobile satellite communication system. A earth channel base station 6 is a base station of the cellular system. A satellite channel base station 7 is a base station of the mobile satellite communication system. The mobile communication terminal unit 1' comprises antennas 10.sub.1 and 10.sub.2, a satellite channel RF portion 20.sub.1, and a cellular channel RF portion 20.sub.2. The antennas 10.sub.1 and 10.sub.2 operate corresponding to the two different mobile communication systems. Each of the satellite channel RF portion 20.sub.1 and the cellular channel RF portion 20.sub.2 performs a power amplifying operation, a frequency converting operation, and a modulating/demodulating operation. The mobile communication terminal unit 1' further comprises a switch 30', a controlling portion 40', and a signal processing portion 70.
Next, a mode switching operation of the mobile satellite communication system mode and the cellular system mode will be described. Now, it is assumed that the priority of the cellular system is higher than that of the mobile satellite communication system. In addition, it is assumed that the mobile communication terminal unit 1' is communicating using the mobile satellite communication system. At this point, as the RF portion of the mobile communication terminal unit 1', the satellite channel RF portion 20.sub.1 corresponding to the mobile satellite communication system is used. On the other hand, the cellular channel RF portion 20.sub.2 monitors the signal level of a radio wave received from the earth channel base station 6 so as to determine whether or not the mobile communication terminal unit 1' is in the service area of the cellular system. When the cellular channel RF portion 20.sub.2 has determined that the mobile communication terminal unit 1 is in the service area of the cellular system, after completing a call in the mobile satellite communication system, the switch 30' of the mobile communication terminal unit 1' switches the satellite channel RF portion 20.sub.1 to the cellular channel RF portion 20.sub.2. Thus, the mobile communication terminal unit of the mobile communication network that integrates a plurality of mobile communication systems has hardware portions (RF portions) that are switched corresponding to higher priority systems as disclosed in Japanese Patent Laid-open Publication No. 59-97238.
However, in the mobile communication terminal unit corresponding to a plurality of mobile communication systems, because of hardware portions corresponding to higher priority modes, the size and power consumption of the mobile communication terminal unit increase and the fabrication cost thereof increases. On the other hand, if the hardware portion (such as a power amplifying portion, a frequency converting portion, and a modulating/demodulating portion) is shared by a plurality of communication systems, while the mobile communication terminal unit is in a standby mode of a particular communication system, the hardware portion cannot detect whether or not the mobile communication terminal unit is in the service area of another communication system.